The lithographic process comprises several steps which include coating a thin film of the radiation sensitive resist onto a substrate which is selectively image-wise exposed to radiation that defines a pattern on the coating. In so doing, a solubility difference is induced between the exposed and unexposed areas and the developer used in the pattern development process is capable of distinguishing between the two regions and preferentially removing one kind or the other. When the exposed areas are removed by the developer, the remaining pattern is positive while the opposite leads to negative patterns.
The manufacture of integrated circuits and other patterned devices relies primarily on resist materials that enable the formation of high resolution patterns. In the search for materials and methods for formation of patterns below 0.25 microns, it is recognized that such patterns require exposure sources based on UV radiation below 248 nm, or x-ray, or e-beam. In the case of the UV radiation, it might be convenient to use excimer laser sources that produce radiation at 193 nm. Likewise, it is essential to employ resist materials suitable for use with short wavelengths sources.
Moreover, in microcircuits, the critical levels are typically manufactured with negative resists. These levels are found primarily in the gate regions of the transistors, where precise critical dimensions are needed. Since the speed performance of the transistor is directly related to the gate size, it is important to maintain uniform dimensions throughout the circuit, otherwise the timing of the interactions between the components is thrown off and the device malfunctions.
It seems that the need for negative resists at these manufacturing levels is needed because of the difference in performance of positive resists and negative resists with isolated and nested lines. From experience with resists and processes used in CMOS, it is known that some positive resists overemphasize nested lines, while negative resists overemphasize (by much less) isolated lines. The real problem comes from the follow-up step which is a reactive ion etching. In this step there is over etching in isolated lines. Thus, if positive resist is used, then the error is compounded, and conversely when negative resist is used, the error is cancelled. This leads to small ACLV (Across Chip Linewidth Variation).
In the case of memory chips, the circuitry is very repetitive and redundant. There, one can compensate the mask to account for the problems mentioned above and thus use positive resists. However, in logic circuits the patterns are very diverse and such compensation on the mask is impractical. Therefore the critical levels in logic circuits depend upon negative resists.
Another level that is preferentially manufactured with negative resists is the first level of metallization. This level is performed by etching a film of metal. The use of negative resists at this level is due to thin film effects and especially reflective notching from underlying layers which is reduced by negative resists. These resists also have their better thermal stability which is needed during etching processes, and better performance when contamination and mask defects are considered.
A third level where use of negative resists is customary but not critical involves making of alignment marks.
In the manufacture of CMOS, there are additional levels that are manufactured with negative resists. These are "Block Levels" which are not regarded as "critical". The use of negative resists in these levels is motivated by the empirical observation that these resists contain less metal ions than positive resists and for those levels this is an important consideration.
The market for negative resists is smaller than the positive resists by a considerable margin (5:1 or so). But their importance to the manufacturing process cannot be overestimated, particularly in view of shrinking dimensions in future device generations. Negative resists have been widely discussed recently. Commercial formulations that incorporate base soluble polymers, acid sensitive crosslinking agents and photoacid generators have been introduced to the marketplace. However, it would be desirable to provide a negative resist capable of operating at 193 nm UV radiation.